Well, we know that Cassandra didn't go so far as to poison Roger, because I don't remember there being any slow-acting poison on Dark Shadows. It's all instant gratification. But why didn't Cassandra poison Roger? It would have made her existence so simple! She could be the grieving widow, living at Collinwood so as to soak up the atmosphere of the place where her beloved late husband had spent his life (outside of those idyllic years in Augusta, which seem to have been conveniently forgotten). Instead she chose to keep him alive and miserable.
And she didn't need to be so spiteful to Elizabeth, either. She could have expressed shock and anxiety upon hearing of Roger's mental problems, and said that she had seen no sign of them, without suggesting that Elizabeth was a delusional control freak.
So one can only conclude that Cassandra truly enjoys creating discord for its own sake. Angelique in 1795 - before her death, I mean - did not. Everything was aimed at inducing Barnabas to love her of his own free will, and making sure she wasn't accused of being a witch. Nothing else was worth her time. But since then, Diabolos her master has clearly explained to her that her own aims are not enough: she must also promote the greater evil.
Why didn't it occur to Vicky that she herself might be Cassandra's target? Vicky was there in 1795, and Angelique put in her two cents to make sure that Vicky was convicted of witchcraft. We know that Vicky is not Cassandra's target - but only because we, unlike Vicky, know that the Barnabas of 1795 and the Barnabas of 1967 are one and the same.
